gh-91052: Add PyDict_Unwatch for unwatching a dictionary (#98055)

Author: carljm

Doc/c-api/dict.rst

@@ -246,24 +246,41 @@ Dictionary Objects
    of error (e.g. no more watcher IDs available), return ``-1`` and set an
    exception.
 
+   .. versionadded:: 3.12
+
 .. c:function:: int PyDict_ClearWatcher(int watcher_id)
 
    Clear watcher identified by *watcher_id* previously returned from
    :c:func:`PyDict_AddWatcher`. Return ``0`` on success, ``-1`` on error (e.g.
    if the given *watcher_id* was never registered.)
 
+   .. versionadded:: 3.12
+
 .. c:function:: int PyDict_Watch(int watcher_id, PyObject *dict)
 
    Mark dictionary *dict* as watched. The callback granted *watcher_id* by
    :c:func:`PyDict_AddWatcher` will be called when *dict* is modified or
-   deallocated.
+   deallocated. Return ``0`` on success or ``-1`` on error.
+
+   .. versionadded:: 3.12
+
+.. c:function:: int PyDict_Unwatch(int watcher_id, PyObject *dict)
+
+   Mark dictionary *dict* as no longer watched. The callback granted
+   *watcher_id* by :c:func:`PyDict_AddWatcher` will no longer be called when
+   *dict* is modified or deallocated. The dict must previously have been
+   watched by this watcher. Return ``0`` on success or ``-1`` on error.
+
+   .. versionadded:: 3.12
 
 .. c:type:: PyDict_WatchEvent
 
    Enumeration of possible dictionary watcher events: ``PyDict_EVENT_ADDED``,
    ``PyDict_EVENT_MODIFIED``, ``PyDict_EVENT_DELETED``, ``PyDict_EVENT_CLONED``,
    ``PyDict_EVENT_CLEARED``, or ``PyDict_EVENT_DEALLOCATED``.
 
+   .. versionadded:: 3.12
+
 .. c:type:: int (*PyDict_WatchCallback)(PyDict_WatchEvent event, PyObject *dict, PyObject *key, PyObject *new_value)
 
    Type of a dict watcher callback function.
@@ -289,3 +306,5 @@ Dictionary Objects
    If the callback returns with an exception set, it must return ``-1``; this
    exception will be printed as an unraisable exception using
    :c:func:`PyErr_WriteUnraisable`. Otherwise it should return ``0``.
+
+   .. versionadded:: 3.12

Doc/whatsnew/3.12.rst

@@ -546,6 +546,11 @@ New Features
   which sets the vectorcall field of a given :c:type:`PyFunctionObject`.
   (Contributed by Andrew Frost in :gh:`92257`.)
 
+* The C API now permits registering callbacks via :c:func:`PyDict_AddWatcher`,
+  :c:func:`PyDict_AddWatch` and related APIs to be called whenever a dictionary
+  is modified. This is intended for use by optimizing interpreters, JIT
+  compilers, or debuggers.
+
 Porting to Python 3.12
 ----------------------
 

Include/cpython/dictobject.h

@@ -106,3 +106,4 @@ PyAPI_FUNC(int) PyDict_ClearWatcher(int watcher_id);
 
 // Mark given dictionary as "watched" (callback will be called if it is modified)
 PyAPI_FUNC(int) PyDict_Watch(int watcher_id, PyObject* dict);
+PyAPI_FUNC(int) PyDict_Unwatch(int watcher_id, PyObject* dict);

Lib/test/test_capi.py

@@ -20,6 +20,7 @@
 import weakref
 from test import support
 from test.support import MISSING_C_DOCSTRINGS
+from test.support import catch_unraisable_exception
 from test.support import import_helper
 from test.support import threading_helper
 from test.support import warnings_helper
@@ -1421,6 +1422,9 @@ def assert_events(self, expected):
     def watch(self, wid, d):
         _testcapi.watch_dict(wid, d)
 
+    def unwatch(self, wid, d):
+        _testcapi.unwatch_dict(wid, d)
+
     def test_set_new_item(self):
         d = {}
         with self.watcher() as wid:
@@ -1477,27 +1481,24 @@ def test_dealloc(self):
             del d
             self.assert_events(["dealloc"])
 
+    def test_unwatch(self):
+        d = {}
+        with self.watcher() as wid:
+            self.watch(wid, d)
+            d["foo"] = "bar"
+            self.unwatch(wid, d)
+            d["hmm"] = "baz"
+            self.assert_events(["new:foo:bar"])
+
     def test_error(self):
         d = {}
-        unraisables = []
-        def unraisable_hook(unraisable):
-            unraisables.append(unraisable)
         with self.watcher(kind=self.ERROR) as wid:
             self.watch(wid, d)
-            orig_unraisable_hook = sys.unraisablehook
-            sys.unraisablehook = unraisable_hook
-            try:
+            with catch_unraisable_exception() as cm:
                 d["foo"] = "bar"
-            finally:
-                sys.unraisablehook = orig_unraisable_hook
+                self.assertIs(cm.unraisable.object, d)
+                self.assertEqual(str(cm.unraisable.exc_value), "boom!")
             self.assert_events([])
-        self.assertEqual(len(unraisables), 1)
-        unraisable = unraisables[0]
-        self.assertIs(unraisable.object, d)
-        self.assertEqual(str(unraisable.exc_value), "boom!")
-        # avoid leaking reference cycles
-        del unraisable
-        del unraisables
 
     def test_two_watchers(self):
         d1 = {}
@@ -1522,11 +1523,38 @@ def test_watch_out_of_range_watcher_id(self):
         with self.assertRaisesRegex(ValueError, r"Invalid dict watcher ID 8"):
             self.watch(8, d)  # DICT_MAX_WATCHERS = 8
 
-    def test_unassigned_watcher_id(self):
+    def test_watch_unassigned_watcher_id(self):
         d = {}
         with self.assertRaisesRegex(ValueError, r"No dict watcher set for ID 1"):
             self.watch(1, d)
 
+    def test_unwatch_non_dict(self):
+        with self.watcher() as wid:
+            with self.assertRaisesRegex(ValueError, r"Cannot watch non-dictionary"):
+                self.unwatch(wid, 1)
+
+    def test_unwatch_out_of_range_watcher_id(self):
+        d = {}
+        with self.assertRaisesRegex(ValueError, r"Invalid dict watcher ID -1"):
+            self.unwatch(-1, d)
+        with self.assertRaisesRegex(ValueError, r"Invalid dict watcher ID 8"):
+            self.unwatch(8, d)  # DICT_MAX_WATCHERS = 8
+
+    def test_unwatch_unassigned_watcher_id(self):
+        d = {}
+        with self.assertRaisesRegex(ValueError, r"No dict watcher set for ID 1"):
+            self.unwatch(1, d)
+
+    def test_clear_out_of_range_watcher_id(self):
+        with self.assertRaisesRegex(ValueError, r"Invalid dict watcher ID -1"):
+            self.clear_watcher(-1)
+        with self.assertRaisesRegex(ValueError, r"Invalid dict watcher ID 8"):
+            self.clear_watcher(8)  # DICT_MAX_WATCHERS = 8
+
+    def test_clear_unassigned_watcher_id(self):
+        with self.assertRaisesRegex(ValueError, r"No dict watcher set for ID 1"):
+            self.clear_watcher(1)
+
 
 if __name__ == "__main__":
     unittest.main()

Modules/_testcapimodule.c

@@ -5296,6 +5296,20 @@ watch_dict(PyObject *self, PyObject *args)
     Py_RETURN_NONE;
 }
 
+static PyObject *
+unwatch_dict(PyObject *self, PyObject *args)
+{
+    PyObject *dict;
+    int watcher_id;
+    if (!PyArg_ParseTuple(args, "iO", &watcher_id, &dict)) {
+        return NULL;
+    }
+    if (PyDict_Unwatch(watcher_id, dict)) {
+        return NULL;
+    }
+    Py_RETURN_NONE;
+}
+
 static PyObject *
 get_dict_watcher_events(PyObject *self, PyObject *Py_UNUSED(args))
 {
@@ -5904,6 +5918,7 @@ static PyMethodDef TestMethods[] = {
     {"add_dict_watcher", add_dict_watcher, METH_O, NULL},
     {"clear_dict_watcher", clear_dict_watcher, METH_O, NULL},
     {"watch_dict", watch_dict, METH_VARARGS, NULL},
+    {"unwatch_dict", unwatch_dict, METH_VARARGS, NULL},
     {"get_dict_watcher_events", get_dict_watcher_events, METH_NOARGS, NULL},
     {NULL, NULL} /* sentinel */
 };

Objects/dictobject.c

@@ -5720,23 +5720,47 @@ uint32_t _PyDictKeys_GetVersionForCurrentState(PyDictKeysObject *dictkeys)
     return v;
 }
 
+static inline int
+validate_watcher_id(PyInterpreterState *interp, int watcher_id)
+{
+    if (watcher_id < 0 || watcher_id >= DICT_MAX_WATCHERS) {
+        PyErr_Format(PyExc_ValueError, "Invalid dict watcher ID %d", watcher_id);
+        return -1;
+    }
+    if (!interp->dict_watchers[watcher_id]) {
+        PyErr_Format(PyExc_ValueError, "No dict watcher set for ID %d", watcher_id);
+        return -1;
+    }
+    return 0;
+}
+
 int
 PyDict_Watch(int watcher_id, PyObject* dict)
 {
     if (!PyDict_Check(dict)) {
         PyErr_SetString(PyExc_ValueError, "Cannot watch non-dictionary");
         return -1;
     }
-    if (watcher_id < 0 || watcher_id >= DICT_MAX_WATCHERS) {
-        PyErr_Format(PyExc_ValueError, "Invalid dict watcher ID %d", watcher_id);
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    if (validate_watcher_id(interp, watcher_id)) {
+        return -1;
+    }
+    ((PyDictObject*)dict)->ma_version_tag |= (1LL << watcher_id);
+    return 0;
+}
+
+int
+PyDict_Unwatch(int watcher_id, PyObject* dict)
+{
+    if (!PyDict_Check(dict)) {
+        PyErr_SetString(PyExc_ValueError, "Cannot watch non-dictionary");
         return -1;
     }
     PyInterpreterState *interp = _PyInterpreterState_GET();
-    if (!interp->dict_watchers[watcher_id]) {
-        PyErr_Format(PyExc_ValueError, "No dict watcher set for ID %d", watcher_id);
+    if (validate_watcher_id(interp, watcher_id)) {
         return -1;
     }
-    ((PyDictObject*)dict)->ma_version_tag |= (1LL << watcher_id);
+    ((PyDictObject*)dict)->ma_version_tag &= ~(1LL << watcher_id);
     return 0;
 }
 
@@ -5759,13 +5783,8 @@ PyDict_AddWatcher(PyDict_WatchCallback callback)
 int
 PyDict_ClearWatcher(int watcher_id)
 {
-    if (watcher_id < 0 || watcher_id >= DICT_MAX_WATCHERS) {
-        PyErr_Format(PyExc_ValueError, "Invalid dict watcher ID %d", watcher_id);
-        return -1;
-    }
     PyInterpreterState *interp = _PyInterpreterState_GET();
-    if (!interp->dict_watchers[watcher_id]) {
-        PyErr_Format(PyExc_ValueError, "No dict watcher set for ID %d", watcher_id);
+    if (validate_watcher_id(interp, watcher_id)) {
         return -1;
     }
     interp->dict_watchers[watcher_id] = NULL;